This invention relates generally to acoustic well logging methods and apparatus and, more particularly, to an improved method and apparatus for acoustically determining bonding characteristics of cement surrounding a cased borehole penetrating an earth formation.
When an earth borehole is drilled for the purpose of producing oil, gas, or other fluids, it is common practice to set casing in the borehole before the production begins. To set the casing, after the casing has been placed in the borehole, cement is pumped into the annulus between the casing and the borehole formation. Optimally, the cement bonds both to the casing and to the formation, thereby holding the casing securely in place and also sealing the earth formations, isolating the different strata in the formation from one another. This isolation prevents the migration of fluids from one strata to another and allows production to be obtained from only those strata or zones as are desired. This production is then achieved by perforating the casing and surrounding cement annulus at the depth of the desired zone, thereby providing a means for any fluid contained therein to enter the casing.
For various reasons, when the cement is pumped into the annulus around the casing, the cement may fail to fully occupy the annulus, thereby defeating the desired effects described above. It is desirable, therefore, to have information indicating the quality of the cement bonding, both to the casing and to the surrounding formation so that remedial actions may be taken if warranted. This information is obtained by operations known commonly in the industry as cement bond logging.
A common technique of cement bond logging involves the use of a logging instrument containing both a transmitter and a receiver of acoustic energy. The transmitter releases acoustic energies into the borehole and thereby into the casing and the surrounding formation. The acoustic energies traverse these mediums until they reach the receiver which transforms the energies into a signal composed of a series of electrical alternations. These alternations will vary in amplitude and form depending upon the route traversed by the acoustic energies from the transmitter to the receiver. In most cases the energies which first arrive at the receiver will be those which traverse the casing rather than those acoustic energies which traverse the formation. Where the casing is free, i.e., unbonded to the cement, the acoustic energies will traverse the casing freely with little attenuation and thus the signal alternations from the receiver representing these acoustic energies first traversing the casing, such alternations being commonly known as casing arrivals, will be at a maximum relative amplitude. However, where the casing is securely bonded to the cement, the acoustic energies traversing the casing will be greatly attenuated and the alternations representative thereof comparatively small. Similarly, intermediate degrees of bonding are reflected in the receiver signal by amplitudes of proportionally intermediate excursion.
Cement bond logging techniques typically also include indications of the alternations representative of later energies which typically are those traversing the surrounding formations between the transmitter and the receiver. These later alternations, commonly known as formation arrivals, may be used to evaluate the quality of the cement bond to the earth formation. A lack of cement bonding to the formation will limit the excursion of the acoustic energy to the formation and there will be virtually no formation arrivals detected by the receiver. Partial cement-formation bonding will usually be evidenced by a reduction in number and amplitude of the formation arrivals as compared to formation arrivals observed under conditions of good cement-formation bonding.
It will be noted that occasionally formations are encountered which exhibit qualities of exceptionally high velocity acoustic energy transfer due to conditions such as exceptional hardness or compaction of the formation. When logging these formations the formation arrivals may either precede or occur simultaneously with the casing arrivals, thereby interfering with and obscuring the amplitudes of the casing arrivals. Because signal alternations which either occur prior to the expected casing arrivals or have an amplitude greater than the amplitudes typical of free casing are indicative of such high velocity formations, where this phenomenon occurs it is typically determinable from the data taken of the formation arrivals. The relative degree of casing-cement bonding is typically not a critical parameter in such a situation because the fast travel of the acoustic energy to and from the formation is accomplished only by good acoustic coupling and is therefore evidence of an acceptable level of casing-cement bonding.
Prior art has attempted to determine the quality of the bonding of the cement to the casing by measuring the amplitude excursion of the signal alternations evidencing the arrival of the casing energies. The amplitude excursion of one or more of these early signal alternations is used as the measure of the attenuation of the acoustic energies traversing the casing and therefore of the degree of casing-cement bonding. Commonly in cement bond logging operations, a number of these signals are generated as the instrument traverses the borehole. The relative amplitude of at least one of the casing arrivals of each of these signals is then commonly displayed as a continuous curve which is correlated to the depth of the instrument within the well while the waveforms of the formation arrivals are graphically presented at selected depth intervals. In U.S. Pat. No. 3,401,773, issued to Synnott, a method is disclosed of cement bond logging by means of acoustic energy impulses as discussed above, and, for determining cement bonding to casing specifically by examination of the amplitude excursion of a first significant alternation or casing arrival of the receiver signal. A similar type of operation also employing an amplitude excursion measurement is disclosed in U.S. Pat. No. 3,292,146, issued to Dewan.
One problem with this amplitude excursion type of measurement is inaccuracies introduced into the measurement by signal noise. In addition to noise due to digital processing and generally high gain-type amplifiers, a source of noise inherent in cement bond logging operations is "road noise" caused by the centralizers used to position the logging instrument centrally within the well. As the logging instrument is caused to traverse the well, the sliding of the centralizers against the casing generates acoustic energies detectable by the receiver within the instrument. The noise then becomes an undesirable component of the receiver signal. The signal noise can either increase or decrease the amplitude of the actual signal at any given point within that signal. It can be readily appreciated that when the maximum amplitude excursion of any given portion of the receiver signal is used as the measure of the attenuation of these casing-traversing energies, an alteration in that maximum amplitude excursion due to this noise creates an erroneous indication of the degree of such attenuation, yielding a functionally erroneous indication of the degree of casing-cement bonding.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a reliable method and apparatus for surveying a casing-cement bond, such method and apparatus minimizing inaccuracies caused by the effects of signal noise upon the receiver signal.